The objectives of this project are to identify and characterize the factors that govern herpes simplex virus (HSV) gene expression in transformed cells. Much of our work has been done with cells transformed by the acquisition of the gene for viral thymidine kinase (tk). We have found that expression of this gene remains subject to regulation by products of superinfecting tk-HSV. It seems unlikely, however, that HSV gene products regulate expression of tk in the non-super-infected cell. Clonal derivates of the transformed cells can retain the HSV tk gene and yet express widely different levels of viral tk. The phenotypically tk ion transformed cells are capable of producing greater quantities of infectious HSV than do the phenotypically tk- transformants or untransformed cells. We are currently quantitating the expression of other HSV gene products in these cells by the use, in immunoprecipitation experiments, of antisera prepared in hamster against syngeneic transformed hamster cells.